Field of the Invention
The present invention relates to an anti-reflection film, an optical element including the same, and an optical system.
Description of the Related Art
An anti-reflection film has hitherto been formed on surfaces of an image capturing lens to be used in an image pickup apparatus such as a video camera, photographic camera, or a television camera, in order to increase an amount of transmitted light and avoid ghost, flare, and the like caused by unnecessary light. The anti-reflection film often includes a multi-layer film in which a plurality of dielectric thin films are laminated.
In general, as the anti-reflection film has a larger number of layers laminated therein, its reflection preventing effect becomes higher. It is also effective for the anti-reflection film to use as its outermost layer (uppermost layer) a low-refractive-index layer in order to obtain a high reflection preventing effect. As such low-refractive-index layer, there has been known a so-called porous layer, in which fine pores each having a size equal to or less than a wavelength of light are incorporated into a medium containing SiO2 (silica), MgF2 (magnesium fluoride), or the like. Such porous layer can achieve a substantial reduction in refractive index depending on an air ratio in the medium. When the porous layer has a pore ratio of about 50%, the layer can realize a refractive index of about 1.25. An anti-reflection film using such porous layer has hitherto been known (Japanese Patent Application Laid-Open No. 2009-157264).
Japanese Patent Application Laid-Open No, 2009-157264 discloses an anti-reflection film using as its uppermost layer a porous layer containing as a main component of and having a refractive index of 1.270. Specifically, the anti-reflection film has a structure in which seven layers are laminated on a substrate, and in which, from a substrate side, a layer containing as a main component alumina is formed as a first layer, and high-refractive-index layers having refractive indices of 1.95 or more and 2.25 or less and low-refractive-index layers having refractive indices of 1.35 or more and 1.50 or less are alternately laminated as a second and the following layers. Then, the porous layer (very-low-refractive-index layer) having a refractive index of 1.270 is disposed thereon as the uppermost layer. Thus, a high-performance reflection preventing effect of a reflectivity of 0.3% or less at a wavelength of 550 nm and a reflectivity of 0.6 or more and 1.3% or less at a wavelength of 700 nm is obtained.
The anti-reflection film disclosed in Japanese Patent Application Laid-Open. No 2009-157264 has low reflectivity as compared to related-art anti-reflection films each using SiO2, MgF2 or the like for its outermost layer. However, its reflectivity in a wide wavelength range is not always enough for an anti-reflection film for an image capturing lens in recent years. For example, in order to effectively avoid ghost based on reflection at a sensor surface, ghost based on reflection at a total reflection surface, and the like, the reflectivity required to be reduced in a wide wavelength range.
In order to reduce the reflectivity in a wide wavelength range and thus achieve a good reflection preventing function, it is important to appropriately set configurations such as the number of dielectric thin film layers formed on the substrate, the refractive indices of the respective layers, and the low-refractive-index layer as the uppermost layer. When such configurations are inappropriate, it is difficult to achieve a good reflection preventing effect in a wide wavelength range.